(a) Field of the Invention
The present invention relates to a telephoto zoom lens system.
(b) Description of the Prior Art
There are many known telephoto zoom lens systems whose field angle in the wide position is 30.degree. to 38.degree. and zoom ratio is about 3. Out of said type of zoom lens system, typical ones are disclosed in Japanese published unexamined patent applications No. 131852/78, No. 42208/81, etc. The known zoom lens systems disclosed in said patent applications respectively comprise four lens groups, i.e., a positive lens group, negative lens group, a positive lens group and a positive lens group. Said known zoom lens systems have the following featurs, i.e., each of them is arranged as a lens system wherein the first up to the third lens groups constitute a vari-focal system and the fourth lens group which serves as an imaging lens is arranged in rear of said vari-focal system and wherein respective lens groups have functions independent of each other, said known zoom lens systems being typical of telephoto zoom lens systems.
On the other hand, as zoom lens systems quickly come into wide use in these years, there is a strong demand for a zoom lens system with high portability and operability including a smaller size and lighter weight and, moreover, at a lower price.
Under such circumstances, in order to make the lens system small in size, telephoto zoom lens systems are being arranged that the vari-focal system and imaging lens have strong refractive powers, and this causes aggravation of aberrations, for example, which are undesirable for telephoto type lens systems. As a result, the performance of the lens system decreases and it is difficult to obtain satisfactorily favourable performance over the whole range of focal length of the lens system. Therefore, it is said that the telephoto ratio about 0.9 is the limit which can be attained when trying to make a telephoto zoom lens system small in size.
Besides the known zoom lens systems comprising four lens group for which it is tried to make the lens system small in size by arranging that the vari-focal system and imaging lens thereof have strong refractive powers as described in the above, there are known zoom lens systems which are made small in size by another method, i.e., by adopting a type of zooming with which the overall length of the lens system becomes short. Said type of zooming was widely adopted for zoom lens systems for 8 mm cinecameras. The zoom lens systems disclosed in Japanese published examined patent application No. 32740/73, Japanese published unexamined patent applications No. 172310/82 and No. 199313/83, etc. are known as the zoom lens systems wherein said type of zooming is applied to photographic lens systems for 35 mm cameras. Said known zoom lens systems are arranged to comprise three lens groups and are arranged that the first and second lens groups thereof constitute a vari-focal system and that the third lens group thereof, which is located in rear of the vari-focal system, is kept fixed in respect to the image surface at the time of zooming and is used as an imaging lens.
The zoom lens systems wherein said type of zooming is adopted have advantages that the lens system can be made satisfactorily small in size in the wide position and that the cost of production can be made low because the number of lenses constituting the lens system can be made comparatively small.
However, said known zoom lens systems have a disadvantage that the optical performance is not satisfactorily stable. It seems that the above-mentioned disadvantage is due to the character inherent to said type of zooming. That is, to make the overall length of the lens system short in the wide position and to maintain the zoom ratio as about 3 at the same time, it is necessary to make the distance between the second and third lens groups short in the wide position by making the refractive power of the second lens group strong in the state that the refractive power of the first lens group is made weak and, moreover, to distribute refractive powers to respective lens groups so that the amount of movement of the second lens group at the time of zooming becomes small. Therefore, to obtain the desired zoom ratio, it is unavoidable to make the amount of movement of the first lens group at the time of zooming large. As the aperture stop of said lens system is located in the third lens group which is kept fixed at the time of zooming and of which the imaging magnification is approximately constant, the distance from the front surface of the lens system to the entrance pupil becomes long when the lens system is zoomed from the wide position to the teleposition by moving the first lens group largely. As a result, the variation of aberrations at the time of zooming becomes conspicuous. To say concretely, considerably undercorrected astigmatism and considerable pincushion distortion remain in the teleposition, there is a tendency that the intensity of light in the marginal portion becomes insufficient in the range of intermediate focal length to the telephoto range, and it is difficult to photograph an object at a satisfactorily sort distance. Besides, when it is tried to make the aperture ratio large, heights of paraxial rays become high when passing the second and third lens groups and it becomes difficult to correct aberrations as well as spherical aberration. Therefore, it is difficult to make the aperture ratio larger than about 1:4.5.